I am not familiar with this model enough to get too specific, but I'll give you some ideas.
Very much advise joining
https://www.facebook.com/groups/phonoland/
if you have a FB account. Ron Rich is very active in there and an expert on
all seeburgs up to mid-70s models. The mech itself does not change much over the years, though, just the control modules and such.
The 'add' solenoid should also reset a 2-trip toggle switch sensor on the front side of the carriage/mechanism, a clever little rocker assembly. This forces two "scan journeys", over to the left and back (twice), and then shuts off the motor - a low-tech way of telling it there's no more songs left to play, so go ahead and shut down, but not without doing at least one more full check, since every time you enter a new selection, the solenoid fires off and resets this 4-position rocker back to zero. Make sure this is engaging and its switches are making/breaking contact correctly. On most models the rocker is reset using a small solenoid.
Can you find the black rubber coupling on the motor? Try spinning this by hand to ensure the gears and such are not binding and the mech will move freely if and when it gets power. If not, you have bad bearings or worse. Poor bearings will transfer motor noise to the amp (extra whirling buzz on top of music). Depending on clutch position, it could be engaged for SCAN but it may also be triggered and trying to LOAD. Undoe a couple large wingbolts on the left and right mounting rails the entire carriage sits on (base frame) and now it will (carefully) slide out so you can poke your head back there and get a good look at the clutch. it almost always is gummed up and needs full clean & lube. there is a V-notched spinning collar that pushes up to engage the LOAD wormgear or DOWN to engage SCAN mode. Center/unengaged position is PLAY.
Check the contact blades and wiring to those switches, too. The manual will lay out the logic for the motor getting powered. You could be meeting two of three conditions, something like that.